This invention relates to electrostatic imaging and in particular, to a new and improved process and apparatus for improving the contrast in visual images produced by depositing toner onto an electrostatic image.
Various types of electrostatic imaging systems are known today. One type is the xerographic copying machine where an electrostatic charge image is produced on a dielectric such as a sheet of paper, after which the dielectric is exposed to a cloud of toner particles with particles being selectively attracted as a function of the charge density to produce a visual image.
The X-ray imaging system sometimes referred to as ionography or electronradiography produces an electrostatic charge image on a dielectric such as a plastic sheet, with the receptor then being exposed to a cloud of toner particles or to a liquid with toner particles suspended therein, to produce the visual image. One such electronradiography system is shown in U.S. Pat. No. 3,774,029.
The systems discussed above provide hard copies or permanent copies with the visual image bonded to the receptor. Another type of imaging system which produces visual images in real time is shown in U.S. Pat. No. 3,965,352. In this type of system, the electrostatic charge image is formed on a surface exposed to a dielectric liquid with the toner particles suspended therein. When an appropriate electric field is produced in the system, toner particles are selectively attracted to the electrostatic charge image producing a toner particle image which can be viewed by reflected light or scattered light. The process of forming the visual image is reversible by reversing the electric field, leaving the systen ready for forming another electrostatic image and subsequent visual image.
All of these imaging systems utilize electrophoretic particles which have a core and a cover or coating. The core normally is a pigment which provides color, typically black for the office copier and white for the real time imaging system. The coating provides the desired electrophoretic characteristics.
The contrast in a visual image depends to a considerable degree on the number of toner particles which form the image. Hence an electrostatic image with a high charge density will produce a higher contrast visual image, i.e., will provide an imaging system with higher gain. Some of the electrostatic imaging systems produce relatively low charge density and considerable work has been performed in developing electrophoretic particles which will be attracted to low charge density areas to provide high particle density and therefore high contrast visual images.
It is an object of the present invention to provide a new and improved process and apparatus for increasing the contrast in the resultant visual image of an electrostatic imaging system.